During the operation of an airplane, the pilots have the responsibility to decide when to command the lift control devices in order to guarantee performance targets and operational requirements. These decisions are often taken during periods of intense workload, when errors are more likely to occur. An inadequate command at the wrong time could potentially cause degradation of safety margins, exceedance of structural limits, or an aerodynamic stall. The parameters and limits that the pilot uses to support these decisions are well known, but should be evaluated in conjunction with the particular phase of flight, actual status of the airplane in the flight path and the airport in which it is operating at that time.
Generally speaking, lift control devices including “high-lift devices” are movable or stationary surfaces that increase or decrease lift during some phases or conditions of flight. For example, lift control devices are used in combination with airfoils in order to reduce the takeoff or landing speed by changing the lift characteristics of a wing. Lift control devices are frequently used during the takeoff and initial climb and during the approach and landing phases of flight but may also be used in any low airspeed situation.
Various types of lift control devices commonly used on aircraft including:                flaps        slats        slots        spoilers.        
As used herein, the term “high-lift device” encompasses each of these individually and plural ones of them in combination.
A flap (see FIGS. 1A, 1B) is a movable surface on the trailing edge of the wing. The flap is controlled from the cockpit, and when not in use, fits smoothly into the lower surface of each wing (FIG. 1A). Flaps are primarily used during takeoff and landing. There are different kinds of flaps including e.g., split flaps, Fowler flaps, slotted flaps and Krueger flaps (the latter being positioned on the wing's leading edge). The use of flaps increases the camber and/or the area of a wing and therefore the lift of the wing, making it possible for the speed of the aircraft to be decreased without stalling. This also permits a steeper gliding angle to be obtained as in the landing approach.
Slats are movable surfaces on the leading edge of the wing. When the slat is closed, it forms the leading edge of the wing. When in the open position (extended forward), a slot is created between the slat and the wing leading edge. This allows the aircraft to reach higher angles of attack, though producing a higher coefficient of lift. So, by deploying slats, an aircraft can fly at slower speeds, allowing it to take off and land in shorter distances.
Slots are created by extended forward movement of a slat. Slots are used as a passageway through the leading edge of the wing. At high angles of attack, the air flows through the slot and smooths out the airflow over the top surface of the wing. This enables the wing to pass beyond its normal stalling point without stalling. Greater lift is obtained with the wing operating at the higher angle of attack.
Spoilers are lift control devices that intentionally reduce the lift component of an airfoil in a controlled way. In some designs and configurations, spoilers are used in conjunction with flaps (steep approach mode, for example). The crew may have separate means to control spoilers individually, regardless of their function.
FIG. 1 shows a non-limiting example of a concrete illustrative example, namely the “high-lift” devices of a prior art aircraft. In this particular example, the high-lift devices consist of flaps F and slats S that are incorporated throughout the wings of the aircraft. The slat system controls eight slat (S) surfaces S1-S8 on the leading edge of the wing (four per wing) and the flap system controls four double slotted flap (F) surfaces F1-F4 on the trailing edge (two per wing). As shown in FIGS. 1A, 1B, when the flap is operated, it slides backward on tracks and tilts downward at the same time, thereby increasing wing camber and increasing the effective size of the wing.
By selectively providing additional lift when deployed, the FIG. 1 high-lift devices F, S allow the aircraft to remain or become airborne at low speeds that are not possible to be achieved when the aircraft/wings are in cruise configuration. Operation at such low speeds is necessary during takeoff and landing operations, due to safety issues, and optimization of runway distances requirements. The crew can command the high-lift devices any time they judge it is necessary.
During operation prior to takeoff, the pilot of the aircraft shown in FIG. 1 needs to set the appropriate high-lift devices setting, considering the airport, payload and the atmospheric parameters at the time of the takeoff. This definition is made during the flight planning, using the information provided by the airplane manufacturer in the airplane flight manual (“AFM”) and additional information provided by ground dispatch. Since the high-lift device F, S commands are manually set by the pilot in the FIG. 1 prior art aircraft, a limited number of positions are available, in order to minimize the probability of errors.
FIGS. 2A and 2B show an example of a conventional prior art manual control means (a slat/flaps selecting lever L) for controlling the high-lift devices of the FIG. 1 aircraft. This lever L is located for example on the lower right-hand side of the control console. As FIG. 2A shows, the lever L controls both the slats and the flaps together (on some aircraft, the slats and flaps can be controlled independently). The pilot selects a slat/flap position by lifting a trigger (not shown) below the head of the lever to unlatch the lever, then placing the lifted lever into a desired detented position. In this particular prior art example, there are 7 discrete positions (positions numbered 0-5 plus “Full”).
The FIG. 2B chart shows the available positions of the FIG. 2A lever L and the corresponding deployment angles the slats and flaps take for each position of the lever. As FIG. 2B shows, position 0 of lever L controls a flap position of 0° and a slat position of 0° the flaps F and slats S are fully retracted (“Up”) and form part of the wing airfoil (see FIG. 1A). Moving lever L to position 1 extends the slats S to a 15° position and extends the flaps to a 7° position (see FIG. 1A). Pulling the lever L further toward the “down” indication results in further extension of the slats S and flaps F at predetermined extended positions of 25° for the slats S and 10°, 20° and 37° for the flaps F. The maximum or “Full” position of lever L controls full deployment of the slats S and flaps F to 25° and 37° respectively. Note that this control lever L controls both the slats S and the flaps F together, and positions 4 and 5 control the same slat/flap deployment with the difference being that position 4 is a gated/stop position that the pilot needs to move past by depressing the trigger again to move the lever to the position 5 detent before proceeding to “Full”.
In the example shown, intermediate positions for the high-lift devices are not available for the slat/flap selector lever L. The lever L positions are discrete and detented just like an automatic gear shift lever of an automatic transmission of a car. There are no intermediate positions between for example lever position 0 and lever position 1, or between lever position 1 and lever position 2. If the lever L is left at an intermediate position between the detented positions, slats/flaps S, F remain in the last selected position.
The lever L was designed to have a limited number of available positions in order to minimize the probability of errors and provide a straightforward procedure to the crew. However, this simplification causes the airplane to operate in conditions out of optimum most of the time. The appropriate positions of the high-lift devices are set prior to takeoff, and not during the takeoff run, in order to reduce the number of actions that the pilot should take during this critical phase of the flight. Therefore, when the pilot starts rolling the aircraft down the runway, the additional drag produced by the high-lift devices will be carried throughout the whole takeoff run, increasing the takeoff distance necessary for lift-off.
After the lift-off, the pilot should monitor the speed, rate of climb and altitude. Retraction of the high-lift devices should be commanded when the actual airspeed is above the minimum retraction speed, but below the airspeed for which the high-lift devices structure was designed. This gives to the pilot a small airspeed window to command the high-lift devices, in a period where the pilot workload is still high, mainly due to obstacle clearance and traffic coordination with the control tower. If retraction of the high-lift devices is commanded at lower airspeeds, the airplane could encounter an aerodynamic stall at low altitude, with little space for recovery. If it is commanded at a higher airspeed, the high-lift devices could be structurally damaged or jammed, which could cause an aerodynamic asymmetry and potentially controllability issues.
When returning for landing, the inverse logic applies. The pilot should decelerate the airplane to the appropriate reference landing speed and deploy the high-lift devices. Both are calculated considering the airport and the atmospheric parameters at the time of the landing, using the information provided by the airplane manufacture in the AFM. The pilot should monitor all the parameters and command the high-lift devices' deployment at the appropriate airspeed, in a similar high-workload environment of the takeoff phase (low altitude, obstacle clearance, traffic coordination).
During the landing run, after the touchdown, the airplane needs to decelerate, and this is done with the high-lift devices in the same position used during final approach. It is not current practice for the pilot to command the high-lift devices to retract during this phase mostly due to high workload. The continued extension of the high-lift devices during the landing run after touchdown causes the airplane to use more runway distance for stopping than it might otherwise need to, since the deployed high-lift devices reduce the normal force acting in the landing gear wheels (due to the high-lift), which reduces braking efficiency.
If the pilot needs to abort the landing and go around or even perform a touch-and-go (for any reason), he should apply thrust/power and reconfigure the high-lift devices, while simultaneously watching the airspeed, altitude, rate of climb, obstacle clearance and traffic coordination. Aviation history brings us many examples of accidents that happened at this phase, due to the inability of the pilot to properly handle all that complexity.
Work has been done in the past to provide some degree of automatic control of high-lift devices.
One approach monitors the upper airspeed threshold and automatically retracts the high-lift devices in order to prevent structural damage. The system commands the deployment to the original commanded position when the airspeed is reduced to a compatible value.
In another system, a flight computer is installed in the airplane, and that computer can calculate on board the optimum performance flap, considering all airport and airplane data. Then, the pilot manually sets the flap.
In yet another system, a flight computer is installed in the airplane, and that computer can calculate on board the optimum performance flap for a go around operation, considering the airport being operated and the actual airplane parameters. With this system, when the pilot commands the flap lever to the go-around position, the airplane will automatically set the proper flap for optimum performance.
Another system automatically commands the high-lift systems based on aircraft parameters, but following a pre-selection made by the pilot using a control panel.
None of these known solutions solve the complete problem, which is eliminating the need of pilot action to command the high-lift devices during all flight phases.